1. Field of the Invention
The present invention relates to an encapsulation for a high voltage interrupter, and more preferably, to a method of casting the interrupter in epoxy.
2. Description of Related Art
Vacuum interrupters are well known in the art for interrupting high voltage lines. See U.S. Pat. Nos. 4,839,481; 4,871,888; 4,982,059; and 5,387,772, the subject matter of which are hereby incorporated herein by reference. The contacts of the interrupters are sealed in a vacuum within a ceramic (such as Al.sub.2 O.sub.3) cylinder or bottle.
A prior art switch vacuum bottle is cast within the upper end of an epoxy structure or encapsulation that includes an internal cavity for the operating rod. The mechanical and dielectric properties of the encapsulation were enhanced by molding the Al.sub.2 O.sub.3 bottle in a layer of polyurethane. The polyurethane encased bottle was then cast in an epoxy encapsulation. A problem with the prior art occurs because of the difference between the coefficient of thermal expansion of the epoxy, the coefficient of thermal expansion of the Al.sub.2 O.sub.3 ceramic housing, and the upper thermal limit of the polyurethane.
In particular, the epoxy is typically cured at high temperatures, such as 140.degree. C. As evidenced in FIG. 1, when the epoxy cools, the coefficient of thermal expansion of the epoxy changes dramatically. The changes in the coefficient of thermal expansion of the epoxy cause the epoxy to come under significant tension and stress when cooled because the ceramic housing and the polyurethane coating have much different coefficients of thermal expansion. As a result, cracks or potential weak points may be created in the epoxy or polyurethane casting, thus compromising the necessary dielectric qualities of the entire module.
Before the present invention, to overcome the potential external dielectric weakness of the ceramic bottle, the interrupter was submerged in a tank of oil or gas to ensure that the interrupter was adequately insulated. This was heavy and expensive.
An additional problem with the prior art is the high cost of the polyurethane coating.